Wireless communication networks provide a plurality of different services to users. A user typically has a user equipment, UE, e.g. a mobile phone, a laptop, Personal Digital Assistant, PDA or any other type of terminal be which the user makes use of one or more services offered by the wireless communication network.
The wireless communication network may be based on a variety of different technologies both with regards the Radio Access Network, RAN, and also for the Core Network. One example of such a technology is Long Term Evolution, LTE. Transmissions are organised into radio frames of 10 ms, each radio frame consisting of 10 equally sized subframes of 1 ms, as illustrated in FIG. 1a. LTE uses Orthogonal Frequency-Division Multiplexing, OFDM, in the downlink and Discrete Fourier Transform, DFT-spread OFDM in the uplink. The basic LTE physical communication resources can thus be seen as a time-frequency grid of subframes (time domain) and resource blocks (frequency domain), as illustrated in the example in FIG. 1b, where each resource element corresponds to one subcarrier during one OFDM symbol interval (on a particular antenna port).
The resource allocation in LTE is described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous 15 kHz subcarriers in the frequency domain. Two consecutive resource blocks (in time) represent a resource block pair and correspond to the time interval upon which transmission scheduling operates. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
An LTE subframe normally contains 14 OFDM symbols, where the first OFDM symbols, 1, 2 or 3 are used for transmission of physical control channels and the remaining OFDM symbols are used for physical data channel transmissions. In the example illustrated in FIG. 1b, downlink control channels are mapped on the first OFDM symbol only, so in this particular case the mapping of data can start already at the second OFDM symbol, i.e. data can be mapped on 13 OFDM symbols out of 14 (assuming a normal Cyclic Prefix (CP)).
In addition to transmissions of downlink control and data, Common Reference Signals, CRS, are also transmitted. How CRS are transmitted within a subframe is known by the served user equipment, UE, after initial access to the network or if the serving Radio Base Station, RBS, or evolved Node B, eNB, has configured specific measurements on a dedicated subframe pattern for specific cells, for which the subframes are meant to have CRS present in all possible symbols. The CRS is used for channel estimation, as part of demodulation of data and control channels, as well as for mobility and channel quality measurements. LTE also supports demodulation based on user-specific Demodulation Reference Signals, DM-RS, in which some data resources are used for transmitting DM-RS.
In the downlink, physical data is transmitted via the Physical Downlink Shared Channel, PDSCH, and physical control signals are transmitted via three physical control channels: Physical Control Format Indicator Channel, PCFICH, Physical Downlink Control Channel, PDCCH, and Physical Hybrid-ARQ Indicator Channel, PHICH.
The PCFICH carries information about the length of the control region, which can vary dynamically on a subframe basis. After it has been detected, the user equipment knows the length of the control region and thus in which OFDM symbol the data transmission starts. The PCFICH is always transmitted within the first ODFM symbol of the control region at locations in the time-frequency grid that are known by the served UE(s).
The PDCCH carries an assignment, or a grant, to UE(s). After demodulating the PDCCH and receiving the assignment, the UE knows the physical resources containing the data and also how to demodulate the data. In case of demodulating the PDCCH and receiving a grant, the UE knows the resource blocks to transmit the data within and also how the data shall be modulated and transmitted. When to receive an assignment is in general not known in advance so UEs monitor the PDCCH transmissions in all subframes. The time duration of the PDCCH is the same as the length of the control region.
The PHICH carries Hybrid Automatic Repeat request. HARQ, acknowledgement, ACK/negative acknowledgement, NACK, responses to UEs indicating whether the uplink data transmission in a previous subframe was successfully decoded by the RBS or not. Which physical resources within the control region that carry PHICH are known by UEs after acquiring system information and when to receive a PHICH is given by the time instant of corresponding uplink data transmission. The time duration of the PHICH is either one or three OFDM symbols, depending on cell configuration. In the case of extended PHICH (i.e. durations of three ODFM symbols) UEs may not need to detect PCFICH to acquire the length of the control region.
The physical downlink control channels are mapped in a cell-specific way on resources in the time-frequency grid that span over the whole system bandwidth whereas data channels can be mapped to an arbitrary number of resource blocks within the system bandwidth. The modulation schemes used for PDSCH transmissions are Quadrature Phase Shift Keying, QPSK, 16 Quadrature Amplitude Modulation. QAM, and 64QAM, whereas physical control channels are always transmitted with QPSK modulation. When CRS are used for demodulation of PDSCH, the transmit power differences between CRS and PDSCH need to be known by the UE when data is modulated with 16QAM and 64QAM.
The LTE system has been developed in such a way that reliable communication is possible even with low signal to interference and noise level ratios (Signal to Interference Noise Ratio, SINR), which makes it possible to deploy networks with a frequency reuse factors of 1 (i.e. neighbouring RBSs or cells using the same frequency). However, a frequency reuse of one still implies that UEs near cell edges or cell borders experience more interference as compared to cell centre UEs. As such, co-ordination of the scheduling between neighbouring cells may be beneficial to ensure that even cell edge UEs will get fair share of the overall cell capacity. For example, neighbouring RBSs can choose to use a frequency reuse of 1 only in their central region and apply scheduling restrictions so that they don't use the same frequency resources in their cell borders, basically creating a partial frequency reuse in the cell border areas.
Inter-cell Interference Co-ordination, ICIC, is a mechanism by which RBSs consider the interference from and to neighbouring RBSs in their scheduling decisions. Since the RBSs are fully responsible for their scheduling decisions for performing scheduling, ICIC requires some messages to communicate scheduling and interference situations between neighbouring RBSs. The messages used may be messages of an X2 interface, or in other words messages comprised in an X2 application protocol, X2-AP.
For ICIC in the UL direction in 3rd Generation Partnership Project, 3GPP LTE Release 8, two X2 Information Elements, IEs, are available as part of the X2: LOAD INFORMATION message: UL High Interference Indicator, HII, and UL Interference Overload Indicator, OI. Both OI and HII can be communicated between neighbouring as often as every 20 ms.
The HII is an IE that may be sent by an RBS to its neighbouring RBSs to inform them about the UL Physical Resource Blocks, PRBs, that it is planning to grant to its cell edge UEs in the UL in the near future. The RBSs response to receiving this message is left up to implementation, but one possible reaction could be to refrain for a certain duration from granting the PRBs indicated as interference sensitive in the HII to their cell edge UEs, as those PRBs are expected to experience strong UL interference from the cell edge UEs of the neighbour RBS that sent out the HII message.
The OI is an IE indicating the uplink interference level experienced by a cell on each UL PRBs. Therefore, this IE will typically be sent by an RBS victim of UL interference to an RBS acting as interference aggressor. An aggressor is in this sense an RBS causing interference to a neighbouring RBS, the neighbouring suffering from the interference being the victim. For each PRB, the level of interference can be assigned to low, medium or high. The response to receiving the OI IE is also left up to implementation, but a possible reaction could be for a neighbour RBS to schedule more on the PRBs reported to experience low level of interference and less on the PRBs experiencing high levels of interference until the situation is resolved, for example, neighbour sends out another OI indicating there are few or no PRBs experiencing high interference.
In DL the X2 IE Relative Narrowband Transmit Power, RNTP, indicator has been defined as part of the X2: LOAD INFORMATION message. The RNTP includes a bitmap, where each bit, corresponding to each PRB, indicates whether the RBS is planning to keep the transmit power of the PRB below a certain threshold, known as RNTP threshold, which is also included in the RNTP message. A bitmap value of “0” can be considered as a promise by the RBS not to use a power level higher than the RNTP threshold. The promise is expected to be kept by the cell until a future RNTP message tells otherwise.
The RNTP threshold can take one of these values in dB:
RNTPthreshold∈{—∞, −11, −10, −9, −8, −7, −6, −5, −4, −3, −2, −1, 0, +1, +2, +3}
For example, if the RNTP threshold is −∞, this can be considered as a promise by the RBS to its neighbours that it will not transmit any data on all the PRBs flagged with a “0” bitmap. A threshold value of 0 dB means that less than the nominal transmit power will be used on the PRBs flagged with a “0” bitmap, while a threshold value of +3 indicates that the PRBs flagged with a “1” are actually going to use power boosting up to 50% higher than the nominal transmission power for the cell sending the bitmap.
Similar to the reception of the OI and the HII, the RBS's response to RNTP is left up to implementation. One possible reaction could be for the RBS to avoid scheduling cell edge UEs in the DL on those PRBs expected to be allocated high transmission power by the reporting neighbouring RBS, as they are likely to be the ones to be scheduled to the cell edge UEs of the reporting neighbouring RBS.
Thus the RNTP can be considered as the DL equivalent of the UL HII (but with more information, since the HII doesn't provide any thresholds), as it provides the relative interference to be experienced at particular PRBs.
The ICIC mechanisms described in the previous section are all target to the data region only and currently there is no mechanisms standardized for the control region.